The present invention is directed toward a method and apparatus for accurately determining a coordinate of an antenna relative to a predetermined reference location, in particular an antenna for ground penetrating radar.
Ground penetrating radar (xe2x80x9cGPRxe2x80x9d) systems are used to obtain measurements of subsurface structures and provide images of the internal structure of opaque materials such as soil, rock, concrete, asphalt and wood.
Most GPR equipment utilizes time-domain methods to penetrate a medium. This typically entails generation and radiation of short electromagnetic pulses in a range of 10 MHz to 2 GHz. The radiated pulses propagate from a system""s radar transmitter and transmitting antenna, penetrate the subsurface medium, and reflect, refract and/or diffract at boundaries of intrinsic impedance contrasts, commonly referred to as targets, in the subsurface medium. A portion of the redirected energy propagates back to a receiving antenna, where the energy may be processed, displayed and stored. In this manner, a time versus distance map of a series of measurements over the medium surface can be constructed to provide a cross-sectional image of targets within the medium. Data is often collected along a series of parallel lines to yield a 3-D image of reflectors (targets) in the subsurface medium. The quality of the 3-D image is directly correlated to the accuracy of the (x,y) coordinates of the parallel profile lines.
A conventional method for collecting data along parallel profile lines and demarcating the starting and ending (x,y) coordinates of each profile line is to position the antenna on a grid and move the antenna along each line on the grid. Typically the starting point of each parallel line is indicated by a user-generated mark in the data or by separating the data from each profile line into separate data files. Both methods rely on the user to accurately position the antenna on the starting point of each profile line and to record the (x,y) coordinate of the starting point of each line.
Generally it is desired that the user will traverse along a straight line so that the ending y-coordinate is the same as the starting y-coordinate. However, this is not always the case, particularly if the area being surveyed is large and there is thus a large distance between the beginning and end of a line.
A distance encoder, such as a meter wheel, is often used to record the distance the antennas have traveled and the length of a profile line. If the surface of the area being surveyed is uneven, the wheel may (and often will) skip, thus introducing errors into the distance measurement. In applications where centimeter-level accuracy in the position of the antennas is important, such errors in the distance measurement may be unacceptable. Also, it is often undesirable to rely on the radar operator to position the antennas accurately and to record these positions since there is a likelihood of operator error, particularly when a very high level of positioning accuracy is required and in any event such efforts increase the time involved to successfully collect the data.
There is, accordingly, a need for a method for determining the position of a GPR antenna with high accuracy over a large surface area. Probably such a method would be efficient to employ and require apparatus which is inexpensive and simple to deploy and use.
The foregoing and other needs are addressed by, and advantages accrued from, the new method and apparatus disclosed herein.
According to one embodiment, such a method for determining a coordinate of an antenna relative to a predetermined reference comprises acts of placing transmitting and receiving antennas in proximity to a reflective strip, transmitting a first signal from the transmitting antenna and detecting a reflection of the first signal received by the receiving antenna from the reflective strip. The method further comprises determining a coordinate of at least one of the receiving antenna and the transmitting antenna relative to the reflective strip based on a parameter of the reflection.
The method may further comprise acts of detecting a second signal received by the receiving antenna from a conductive strip located proximate to the reflective strip, and determining a second coordinate of at least one of the receiving antenna and the transmitting antenna based at least on a time difference between transmitting the first signal from the transmitting antenna and receiving the second signal at the receiving antenna.
Another embodiment is directed toward a method for determining a coordinate pair of an antenna relative to a predetermined reference, the method comprising acts of placing the antenna proximate to a structure including first and second conductive strips located on either side of a reflective strip, transmitting a first signal from the antenna, and measuring at least one parameter of a second signal received by the antenna, the second signal corresponding to a reflection of the first signal by the reflective strip. The method further comprises transmitting a third signal from the antenna to the first conductive strip, measuring at least one parameter of the third signal received by the antenna from the second conductive strip, and determining an first coordinate of the antenna based on a measurement of the at least one parameter of the second signal. The method also includes determining a second coordinate of the antenna based on a measurement of the at least one parameter of the third signal.
According to another embodiment, there is provided an apparatus for determining a coordinate of an antenna relative to a predetermined reference. The apparatus comprises a transmitting antenna that transmits a signal, a reflective strip that reflects at least a portion of the signal transmitted by the transmitting antenna, and a receiving antenna that receives a reflected signal from the reflective strip. The apparatus also includes a controller, coupled to the receiving antenna, that measures at least one parameter of the reflected signal and determines the coordinate of at least one of the transmitting antenna and the receiving antenna based on a measurement of the parameter.
In one example, the apparatus may further comprise a conductive strip(s) substantially surrounding the reflective strip. The controller may be adapted to measure at least one parameter of a second signal received from the conductive strip by the receiving antenna, the second signal corresponding to at least a portion of the signal transmitted by the transmitting antenna that travels along the conductive strip. The controller may be further adapted to determine a second coordinate of at least one of the transmitting antenna and the receiving antenna based on a measurement of the at least one parameter of a second signal.
According to yet another embodiment, a system that provides an image of a subsurface area comprises a control unit and an antenna, coupled to the control unit, that transmits signals into a medium and receives reflected signals from the medium, the antenna being moved over the area in parallel lines to provide scans of radar data of the area. The system also includes a reflective strip(s) that reflects at least a portion of a signal transmitted by the antenna to provide a reflected signal. The control unit utilizes at least a portion of the reflected signal received by the antenna to provide an accurate position of the antenna in one dimension, the position being used to align successive scans of radar data to provide the image.
The system may further include a conductive strip, wherein the antenna transmits a signal that propagates on the conductive strip and a portion of the signal is received by the antenna after a time interval. The control unit may determine a position of the antenna in a second dimension based on the time interval.
The transmitting and receiving antennas for any of the above embodiments may be provided as a single antenna that performs both functions.